Thanks so far. So, I created a new section in boards.txt for testing, selected it in IDE, and used fuse calculator from your link. According to 328 manual, CKSEL3..0 for external crystal should be from 1111 to 1010. I tried to set bootloader.low_fuses from F0 to FF with different SUT setting as well. After each change in boards.txt I reopened IDE and reload the Blink sketch. No change - stil working on low frequency.
Any other idea on the cause? What else to check? Thanks.